Shenqi Pill (pills for invigorating kidney qi) of Treatise on Exogenous Febrile Diseases is considered as classical prescriptions based on precise and appropriate compatibility，rigorous composition of prescription，significant clinical effect，a prescription for multiple syndromes，and that，it is sharing the homology of TCM and Book of Changes perfectly. The essence of Shenqi pill is that kidney qi is the hardcore of body according to the detailed analysis，also，primordial which contains Yin and Yang is originated in Water and Fire that firmly hides in kidney. Relationship between Water and Fire is the same as substance and virtue，the Water is getting warm while the Fire can firmly hide attributing to the interaction of each other. The life will be in a circle if the Water and Fire is in the compatible relationship.